This invention relates to telephone equipment and is more specifically directed to an exclusionary circuit or device, namely a device that permits two telephonic instruments or other devices to share a single communication circuit, such as a central office line or trunk. The invention is specifically directed to means whereby a pair of telephone devices share a common central office line without one device interrupting or interfering with the other. In practice, one of the devices can be a facsimile machine or computer modem, and the other can be a voice instrument, key system, or private branch exchange.
It is conventional wisdom to assign a dedicated telephone trunk to a computer modem or to a facsimile machine. This is viewed as necessary because inadvertent extraneous signals on the line can interfere with the transmission or reception of data or documents by the modem or facsimile machine. These extraneous signals can occur if another device, e.g. an extension instrument, comes on line while the facsimile machine or modem is connected. For that reason, a dedicated line is considered more desirable than a shared line. However, in practice, a dedicated trunk or line for a modem or facsimile machine is in use only a fraction of the business day, i.e. one or two percent of the time, or less. It would be highly desirable to use the telephone line for other purposes hen the facsimile machine or modem is not in use. However, if the phone line is shared between the modem or facsimile machine and another communication device, interference or interruptions can result in failed transmission of data or documents, unless an effective exclusion device is employed. Unfortunately, no such device has thus far been proposed or put into use.
A number of line exclusion features have been proposed, specifically in connection with key telephone equipment. Some of these are described in U.S. Pat. Nos. 3,961,144; 4,218,590; and 4,266,103. An automatic electronic disconnector for telephone equipment is described in U.S. Pat. No. 4,444,999. A facsimile machine with an automatic disconnect means is described in U.S. Pat. No. 3,962,534. An arrangement of branched telephone sets to share an incoming telephone line and which incorporates an exclusion feature is described in U.S. Pat. No. 3,435,151.